As a blue-violet semiconductor laser has been put into practical use, a Blu-ray disc (hereafter BD), as a high-density and large-capacity optical information recording medium (hereinafter, also called as an optical disk) and having substantially the same size as a CD (Compact Disc) and a DVD (Digital Versatile Disc), has been put into practical use. BD is an optical disk for recording or reproducing information on or from an information recording surface of which thickness of the light transmission layer is substantially 0.1 mm, using a blue-violet laser source which emits a laser beam having substantially a 400 nm wavelength, and an objective lens of which numerical aperture (NA) is substantially 0.85. In this description, the light transmission layer refers to a layer between the surface of the information recording medium and the information recording surface.
Many types of compatible objective lens are known, which is a single objective lens for recording or reproducing information on or from a plurality of types of optical disks having a different thickness of the light transmission layer.
For example, Patent Literature 1 discloses an objective lens having a first area, a second area and a third area, which are substantially concentric with respect to the optical axis, at least on one surface of the objective lens.
This objective lens records or reproduces information on or from a first optical disk having a transparent substrate (light transmission layer) of which thickness is t1 (t1=0.6 mm), using a laser beam having the wavelength λ1 (λ1=635 nm), which passes through the first area and the third area, and records or reproduces information on or from a second optical disk having a transparent substrate of which thickness is t2 (t2=1.2 mm), using a laser beam having the wavelength λ2 (λ2=780 nm), which passes through the first area and the second area.
If this compatible objective lens is used, information can be recorded on or reproduced from a plurality of types of optical disks, such as a CD and a DVD, having a different thickness of the light transmission layer and a different NA, using one objective lens.
Many types of compatible objective lenses are also known, where a diffraction structure is formed on the objective lens whereby spherical aberration, generated by the difference of thickness of the light transmission layer among a plurality of types of optical disks, is corrected using the difference of the light source wavelength.
For example, Patent Literature 2 discloses an objective lens, which generates a function of a convex lens by diffracting the blue-violet laser beam having the wavelength λ1, so as to converge the focal spot on the information recording surface of the BD of which thickness of the light transmission layer is substantially 0.1 mm, and which generates a function of a concave lens by diffracting the red laser beam having the wavelength λ2, and converges the red laser beam on the information recording surface of the DVD of which thickness of the light transmission layer is substantially 0.6 mm.
FIG. 21 is a diagram depicting a configuration of a conventional objective lens. The left drawing in FIG. 21 is a schematic plan view depicting a configuration of a conventional objective lens 90, and the right drawing in FIG. 21 is a schematic cross-sectional view depicting the configuration of the conventional objective lens 90. A zonal diffraction structure (hologram) is formed on the entrance surface 91 on the light source side (side where laser beam enters) of the objective lens 90, centering around the optical axis OA of the objective lens 90. The diffraction structure is different between the inner circumference area 911 including the optical axis OA and the outer circumference area 912 which is a peripheral area of the inner circumference area 911.
The inner circumference area 911 is a compatible area which is used for recording or reproducing the DVD using a red laser beam, and recording or reproducing the BD using a blue-violet laser beam. The inner circumference area 911 is designed so that the plus first-order diffracted light of the blue-violet laser beam is converged on the information recording surface of the BD, and the minus first-order diffracted light of the red laser beam is converted on the information recording surface of the DVD.
On the other hand, the NA upon recording or reproducing information on or from the BD using the blue-violet laser beam (substantially 0.85) is greater than the NA upon recording or reproducing information on or from the DVD using a red laser beam (substantially 0.60). Therefore it is designed such that the outer circumference area 912 is an area dedicated to the BD, and only the blue-violet laser beam is converged on the information recording surface of the BD, and the red laser beam generates an aberration on the information recording surface of the DVD.
One unit of the step difference of the diffraction structure of the inner circumference area 911 is an amount to generate substantially a 1.25×λ1 [nm] of optical path difference for the blue-violet laser beam having the wavelength λ1 (λ1=405 nm), and the phase modulation amount is π/2 per step. In this case, the diffraction efficiency of the plus first order diffracted light is substantially 80% based on the scalar calculation, which is highest among the orders of diffraction.
One unit of the step difference of the diffraction structure in the inner circumference area 911 is an amount to generate substantially a 0.75×λ2 [nm] of optical path difference for the red laser beam having the wavelength λ2 (λ2=660 nm), and the phase modulation quantity −π/2 per step. In this case, the diffraction efficiency of the minus first-order diffracted light is substantially 80% based on the scalar calculation, which is highest among the orders of diffraction.
If the inner area 911 has this diffraction structure, a compatible recording or compatible reproduction of information can be implemented at high light utilization efficiency for the DVD having a 0.6 mm thick light transmission layer, and for the BD having substantially a 0.1 mm thick light transmission layer.
If a lens having a diffraction structure, which is not limited to a compatible objective lens, is used, the diffraction efficiency may change depending on the radius position of the lens. This is because the pitch of the diffraction structure in the effective diameter of the lens is different depending on the radius position. Generally if the lens power is generated by the diffraction structure, the pitch of the diffraction structure decreases and diffraction efficiency decreases as the radius position approaches from the inner circumference near the optical axis to the outer circumference.
The intensity distribution of the semiconductor laser used for the optical head for the optical disk, on the other hand, decreases Gaussian-functionally as the distance from the optical axis increases, hence the intensity of the laser beam is lower in the outer circumference than in the inner circumference. If the intensity of the laser beam, which enters the objective lens, decreases dramatically in the outer circumference, the effective NA of the objective lens decreases. As a result, the focal spot on the information recording surface of the optical disk cannot be sufficiently focused.
For example, Patent Literature 1 discloses a configuration of a refraction type compatible objective lens that has no diffraction structure, where a second area is a dedicated area for the red laser beam having the wavelength λ2, and a third area is a dedicated area for the infrared laser beam having the wavelength λ1.
However the conventional objective lens is a refraction type compatible objective lens that has no diffraction structure, hence if it is designed to converge the laser beam having the wavelength λ2 on the information recording surface of the CD in the second area, the laser beam having the wavelength λ1 is not converged at all on the information recording surface of the DVD. In other words, absolutely no laser beam having the wavelength λ1 is converged on the information recording surface of the DVD in the second area.
If diffraction efficiency of the laser beam which enters the objective lens becomes completely zero in a part of the area like this, deterioration of the focal spot on the information recording surface becomes enormous.
Furthermore according to the conventional objective lens disclosed in Patent Literature 2, for example, the inner circumference area is used for recording or reproduction for both the DVD and the BD. Therefore the pitch of the diffraction structure becomes small in the area near the outermost circumference of the inner circumference area, and diffraction efficiency drops.
FIG. 22 is a graph depicting a diffraction efficiency of the conventional objective lens. In FIG. 22, in the conventional objective lens 90 disclosed in Patent Literature 2, the diffraction efficiency of the blue-violet laser beam having the wavelength λ1 and that of the red laser beam having the wavelength λ2 are calculated based on the wave calculation (vector calculation). In FIG. 22, the abscissa is the entrance position of the laser beam, that is the distance from the optical axis OA (radius of objective lens), and the ordinate is the diffraction efficiency corresponding to the entrance position.
As FIG. 22 shows, in the case of the wavelength λ2, the diffraction efficiency is 70% or more in a position near the optical axis in the inner circumference area (point α), whereas the diffraction efficiency drops to 50% or less in a position near the outermost circumference of the inner circumference area (point β). This is because the pitch of the diffraction structure is smaller in the position of the point β than the position of the point α, and in addition, the inclination angle of the entrance surface 91 increases and the incidence angle of the laser beam which enters the objective lens 90 substantially parallel increases as the distance from the optical axis OA increases. The diffraction efficiency in the position of the point β can further drop by the dispersion in molding, for example.
If the intensity of the laser beam which enters the objective lens drops dramatically in the outer circumference, as mentioned above, the focal spot on the information recording surface of the optical disk cannot be sufficiently focused, but this problem of the conventional compatible objective lens is not mentioned in Patent Literature 2.